1. Field of the Invention
The invention relates generally to storing of a liquid into a geological formation.
2. Background Art
Storing of a liquid into a geological formation is a common practice, and may be used for example for storing fresh water. The geological formation may for example be an aquifer, i.e. a water bearing stratum of permeable rock, sand or gravel.
The storing of fresh water in an aquifer may be economically more competitive than using tanks, and more particularly surface tanks. This is particularly true when relatively large quantities of water need to be stored for an uncertain amount of time, e.g. in a context of strategic storage. The aquifer is simply used as a reservoir into which the fresh water is placed. This water can be used later: for example in case a normal water providing process is interrupted, or when a drier period occurs and large quantities of water are required, the stored fresh water may be withdrawn. A pump is then placed at the well and the stored fresh water is extracted.
FIG. 1 show a schematic illustration of a water storage system using an aquifer according to prior art. A screen 11 with slots 17 is located on a wall of a well 12 penetrating into the aquifer 13. The slots of the screen enable a flow of liquid. In case fresh water is to be stored in the aquifer, the fresh water is injected from the surface into the well 12 and flows through the screen 11. The fresh water is injected using a pump or any other means as appropriate. The injected fresh water creates a water bubble, i.e., a zone 14 of fresh water inside of the aquifer 13 and extending away from the screen 11. A part of the aquifer surrounding the zone 14 contains native aquifer water. The native water may for example be brackish water 15, i.e. water containing salts. The native water tends to be pushed aside as the volume of the stored fresh water increases.
In the following description it will be assumed as an example that the native water is brackish water.
In case the fresh water is to be extracted from the aquifer, the fresh water flows from the zone 14 through the screen 11 to the surface by flowing inside the well 12. The water is generally pumped to the surface by using a pump placed inside the well. A sensor 16 is used to measure a quality parameter of the extracted water at a level of the surface. The measurements from sensor 16 are used to monitor the quality of the water. Usually the quality of the fresh water may be affected by salt providing from the brackish water 15, or from any other contaminate that is present. This may happen after a part of the stored fresh water has been retrieved and a mixture of fresh water and brackish water created at the border of the zone 14 starts being extracted.
FIG. 2 illustrates an example of a plot of the quality parameter versus time during the extraction of fresh water in the system from FIG. 1. The quality parameter may be a Total Dissolved Salt (TDS) content, or any other parameter used to define water quality. Generally a suitable zone is found so that the fresh water does not move much in the aquifer after it has been injected and during the time in which it is stored. Therefore, at the beginning of extraction, the quality of the extracted fresh water is close to the quality of the injected fresh water: the TDS content has relatively low values as can be seen at the left of the curve in FIG. 2 around the time t0.
As the water continues to be extracted and time increases, the TDS content rises. After a relatively large amount of the injected fresh water has been extracted from the aquifer, the extracted liquid contains an increasing amount of brackish water and the TDS content raises. The TDS content reaches a pre-defined threshold at the time t1. The pre-defined threshold may for example correspond to a maximum tolerable TDS for fresh water. The extraction is then stopped.
The injection of fresh water and its extraction up to a pre-defined TDS content corresponds to an injection-extraction cycle. The injection-extraction cycle may be repeated several times, i.e. the aquifer and its water storing system may be re-used.
A recovery efficiency parameter is defined as a ratio of the volume of extracted water to the volume of injected water during any one cycle. The recovery efficiency parameter increases with the number of injection-extraction cycles. FIG. 3 contains an example curve illustrating the increase of the recovery efficiency parameter. The recovery efficiency parameter is plotted versus the number of injection-extraction cycles. In this example, at the first injection-extraction cycle, the recovery efficiency parameter has a value of about 42%. The efficiency parameter increases with each subsequent cycle and reaches 72% at the end of the fifth injection-extraction cycle.
Often the fresh water is stored seasonally so a cycle may represent 1 year.
In a further embodiment of a water storage system know from prior art, a plurality of wells may be provided. Each well addresses a distinct area of the same aquifer. For each well, the injection-extraction cycle is performed, independently of the other wells. FIG. 4 illustrates an example of a water storage system using a plurality of wells, in which the surface is viewed from above, i.e. from the sky. Each of the wells 42 is represented by a large dot. Each well 42 is used to inject and extract fresh water. As a result of this an extended zone 44 of fresh water is obtained in the aquifer. The fresh water zone 44 is represented by a white surface and is in fact located under the surface in the geological formation. Hatched surfaces represent brackish water under the surface. Hatched surfaces 45 surrounding the fresh water zone 44 represent a transition zone of the aquifer containing brackish water and fresh water, partly mixed.
Further hatched surfaces that are represented as limited surfaces inside the fresh water zone 44 illustrate traps of brackish water 49: these may appear between the wells 42 as a result of operating the wells 42 independently from each other during the injection and extraction of fresh water. During the time that the fresh water is stored in the aquifer, the traps of brackish water 49 may deteriorate the quality of the zone of fresh water 44, thus causing a loss in recovery efficiency.
Similarly, during extraction, each well is used to extract fresh water independently of the other wells. In some cases fresh water located between the wells may not be extracted by any well, thus creating traps of fresh water. This causes a loss of fresh water and reduces the recovery efficiency.